1. Field of the Invention
The present invention relates generally to the platemaking field, and, more particularly, to a high speed vacuum support assembly for supporting a printing plate in the imaging station of a platemaker during the exposure thereof.
2. Description of the Prior Art
In platemaking apparatus, for example, as used in automatic duplicating machines or the like, a photosensitive printing plate is fed to the imaging station of the platemaker, positioned in the image plane of the optical imaging system, and then exposed to an original document. During exposure, it is, of course, imperative that the plate be supported very accurately in the image plane, and a variety of systems are known in the prior art to accomplish this. One of the most common types of systems comprises a vacuum support system wherein the plate is positioned on a support platen and covered by a glass plate to form an air-tight chamber. Air is then withdrawn from the chamber through a plurality of apertures in the support platen, and, as the air is withdrawn, the printing plate will be pressed and held firmly against the support platen for imaging.
There are several disadvantages to such a system. Initially, substantial time is required to fully evacuate the chamber, for example, a couple of minutes. This is because it is necessary that essentially all the air be removed from the chamber to eliminate any air bubbles because the presence of air bubbles can prevent the printing plate from sitting completely flat against the support platen. In addition, because the vacuum is formed between a glass cover plate and the support platen and drawn through the support platen, the glass cover plate will be placed under a significant amount of stress which can result in its being slightly deformed. Since the printing plate is imaged through the glass, this is unacceptable and the prior art frequently employs heavy, thick glass to minimize distortion. This, however, will reduce the amount of light that can pass through the glass which is also an undesirable situation. Furthermore, because the printing plate is pressed against the support platen during imaging, the platen must be optically flat which is relatively difficult to achieve.
Yet, other inadequacies exist in these prior art systems. For one thing, they are not very suitable for systems wherein the imaging station is in a vertical orientation. Also, they are usually not well designed to permit rapid insertion or removal of half-tone screens or other masking plates into the optical system.